In the context of electron and ion transport properties in gas mixtures, the electric field E is rarely used as such. Instead, one uses E/N since most transport properties scale in this quantity, rather than in E itself. In this expression, N stands for the number density.
In an ideal gas at T_0 = 0°C and p_0 = 1013.25 hPa, there are L = 2.6867775 10^19 molecules/cm3, a constant known as Loschmidt's number. For other temperatures and pressures, the number density N can be computed using the ideal gas law: N = p/p_0 T_0/T L. Chamber operation is usually done at temperatures of ~20°C and at atmospheric pressure. The number density under those conditions is 273.15/293.15 × L ~ 2.50 10^19 molecules/cm3.
Since common E fields are in the range of 100-100000 V/cm, E/N expressed in its native units of V.cm^2, would almost invariably have unwieldy exponents. Following a proposal by L.G.H. Huxley et al. [1], E/N is usually expressed using the Townsend unit, written "Td", which equals 10^-17 V.cm^2. Using this convention, 1 Td at usual chamber operation conditions equals about 250 V/cm.
One occasionally comes across the Hx unit, which stands for "Huxley", the first author of the 1966 paper. This unit was introduced by T. Kunst et al. [2] to honour Huxley. It applies a similar scaling to magnetic fields: 1 Hx = 10^17 G.cm^3 such that under normal chamber conditions, 1 Hx equals about 250 G.
Last updated on 10/3/2006.